FLOCK (2)
apply or remove an advisory lock on an open file
SYNOPSIS
#include <sys/file.h>
int flock(int fd , int operation )
DESCRIPTION
Apply or remove an advisory lock on an open file. The file is specified by
fd .
Valid operations are given below:
LOCK_SH
Shared lock. More than one process may hold a shared lock for a given file
at a given time.
LOCK_EX
Exclusive lock. Only one process may hold an exclusive lock for a given
file at a given time.
LOCK_UN
LOCK_NB
Don't block when locking. May be specified (by
or 'ing)
along with one of the other operations.
A single file may not simultaneously have both shared and exclusive locks.
A file is locked (i.e., the inode),
not
the file descriptor. So,
dup (2)
and
fork (2)
do not create multiple instances of a lock.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and
errno
is set appropriately.
ERRORS
EWOULDBLOCK
The file is locked and the
LOCK_NB
flag was selected.
CONFORMING TO
4.4BSD (the
flock (2)
call first appeared in 4.2BSD).
NOTES
flock (2)
does not lock files over NFS. Use
fcntl (2)
instead: that does work over NFS, given a sufficiently recent version of
Linux and a server which supports locking.
flock (2)
and
fcntl (2)
locks have different semantics with respect to forked processes and
dup (2).
SEE ALSO
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